1. Technical Field
The present invention relates to methods for making lead zirconate titanate-based ceramic powders containing lead zirconate titanate, methods for producing piezoelectric ceramics using the same, piezoelectric ceramics produced by the methods, and piezoelectric ceramic elements.
2. Background Art
Piezoelectric ceramics, which have been used for piezoelectric ceramic elements, such as actuators and transformers, are produced using, as raw materials, lead zirconate titanate-based ceramic powders containing as a principal constituent lead zirconate titanate having a large electromechanical coefficient.
In order to synthesize a lead zirconate titanate-based ceramic powder, a so-called solid-phase method is employed in which a lead oxide (PbO) powder, a zirconium oxide (ZrO2) powder, and a titanium oxide (TiO2) powder are weighed, mixed, and pulverized, and then heat treatment is performed.
During the heat treatment, first, from approximately 500xc2x0 C., lead oxide in the lead oxide powder reacts with titanium oxide in the titanium oxide powder to produce lead titanate (PbTiO3), and simultaneously (strictly speaking, from approximately 600 to 700xc2x0 C., when lead titanate is in the process of being generated), lead oxide in the lead oxide powder reacts with zirconium oxide in the zirconium oxide powder to produce lead zirconate (PbZrO3). From approximately 800 to 1,000xc2x0 C., the lead titanate and lead zirconate react to produce lead zirconate titanate (Pb(Zr,Ti)O3).
However, in the heat treatment step described above, in order to obtain lead zirconate titanate having a uniform crystal structure by the reaction of lead zirconate and lead titanate, the produced lead zirconate and lead titanate must be dissolved and made to react homogeneously. That is, once lead zirconate is produced, since the production/dissolution temperature of lead zirconate is high, the synthesis temperature of lead zirconate titanate is increased. If the synthesis temperature is increased, grain growth of lead zirconate titanate grains in the lead zirconate titanate-based ceramic-powder is accelerated, and the grains are coarsened. As a result, since the reactivity of the lead zirconate titanate grains is decreased, when a piezoelectric ceramic is produced by firing the lead zirconate titanate-based ceramic powder, the sintering temperature thereof is increased. If the sintering temperature is increased, the lead component is partially volatilized, and the composition ratio of the resultant sintered compact (piezoelectric ceramic) can easily vary.
The present inventor has disclosed a technique in Japanese Unexamined Patent Application Publication No. 5-306122 in which for the purpose of low-temperature synthesis of a lead zirconate titanate-based ceramic powder, by mixing a fine powder of a monodisperse compound oxide obtained by hydrolysis of alkoxides of constituent elements except lead with a lead oxide powder, followed by heat treatment, a lead zirconate titanate-based ceramic powder can be directly obtained without generating an intermediate compound.
However, in the method in which alkoxides are used as starting materials, the process is complex and troublesome, and the cost of the starting materials themselves is high, so that this method is not ideal for commercial production.
The present invention has been achieved in view of the situation described above, and with the invention it is possible to produce a lead zirconate titanate-based ceramic powder having a uniform, fine crystal structure using a solid-phase method in which a heat treatment temperature is kept low. The present invention is also usable to produce a piezoelectric ceramic of stable quality with less variation in its composition ratio by processing at a low firing temperature, and to provide a piezoelectric ceramic element using the piezoelectric ceramic.
That is, the present invention relates to a method for producing a lead zirconate titanate-based ceramic powder which includes the steps of mixing a lead oxide powder, a zirconium oxide powder, and a titanium oxide powder to form a mixed base powder; and heat-treating the mixed base powder so that lead oxide in the lead oxide powder and titanium oxide in the titanium oxide powder react to produce lead titanate, and that unreacted lead oxide not used in the reaction, the lead titanate produced by the reaction, and zirconium oxide in the zirconium oxide powder react to produce the lead zirconate titanate-based ceramic powder containing lead zirconate titanate. (Hereinafter, this method for producing the lead zirconate titanate-based ceramic powder may be referred to as xe2x80x9ca first production method of the present inventionxe2x80x9d.)
The present invention also relates to a method for producing a lead zirconate titanate-based ceramic powder which includes a first step of mixing and heat-treating a lead oxide powder and a titanium oxide powder so that lead oxide in the lead oxide powder and titanium oxide in the titanium oxide powder react to produce a lead titanate-based powder containing lead titanate; and a second step of mixing and heat-treating the lead titanate-based powder, another lead oxide powder, and a zirconium oxide powder so that lead titanate in the lead titanate-based powder, lead oxide in the other lead oxide powder, and zirconium oxide in the zirconium oxide powder react to produce the lead zirconate titanate-based ceramic powder containing lead zirconate titanate. (Hereinafter, this method for producing the lead zirconate titanate-based ceramic powder may be referred to as xe2x80x9ca second production method of the present inventionxe2x80x9d.)
In accordance with the first or second production method of the present invention, in the heat-treating step for synthesizing lead zirconate titanate, it is possible to suppress the generation of lead zirconate which requires a high-temperature heat treatment, so that the heat-treating temperature required for producing a lead zirconate titanate-based ceramic powder can be kept low. Therefore, the resultant lead zirconate titanate-based ceramic powder has a uniform, fine crystal structure (grain structure) and has superior reactivity. When the lead zirconate titanate-based ceramic powder is used for a piezoelectric ceramic, the amount of unreacted lead oxide being present at the grain boundaries can be decreased.
The present invention further provides a method for making a piezoelectric ceramic using the first production method of the present invention, which includes the steps of: mixing a lead oxide powder, a zirconium oxide powder, and a titanium oxide powder to form a mixed base powder; heat-treating the mixed base powder so that lead oxide in the lead oxide powder and titanium oxide in the titanium oxide powder react to produce lead titanate, and that unreacted lead oxide not used in the reaction, the lead titanate produced by the reaction, and zirconium oxide in the zirconium oxide powder react to produce a lead zirconate titanate-based ceramic powder containing lead zirconate titanate; and forming the lead zirconate titanate-based ceramic powder into a predetermined shape and firing the resultant green compact.
The present invention also provides a method for making a piezoelectric ceramic using the second production method of the present invention, which includes: a first step of mixing and heat-treating a lead oxide powder and a titanium oxide powder so that lead oxide in the lead oxide powder and titanium oxide in the titanium oxide powder react to produce a lead titanate-based powder containing lead titanate; a second step of mixing and heat-treating the lead titanate-based powder, another lead oxide powder, and a zirconium oxide powder so that lead titanate in the lead titanate-based powder, lead oxide in the lead oxide powder, and zirconium oxide in the zirconium oxide powder react to produce a lead zirconate titanate-based ceramic powder containing lead zirconate titanate; and a third step of forming the lead zirconate titanate ceramic powder into a predetermined shape and firing the resultant green compact.
In accordance with each of the methods for making the piezoelectric ceramic, in the step of synthesizing lead zirconate titanate, the generation of lead zirconate can be suppressed, whereby the heat-treating temperature required for producing the lead zirconate titanate-based ceramic powder can be kept low. Furthermore, since the lead zirconate titanate-based ceramic powder has a uniform, fine crystal structure, its reactivity is improved. That is, when the piezoelectric ceramic is formed using the ceramic powder, the firing temperature can be kept low, and the amount of unreacted lead oxide being present at the grain boundaries of the resultant piezoelectric ceramic can be decreased. That is, it is possible to obtain the piezoelectric ceramic which has little variation in its composition ratio, stable quality, and a large electromechanical coefficient.
The present invention also provides a piezoelectric ceramic produced by the method for making the piezoelectric ceramic of the present invention, and furthermore, the present invention provides a piezoelectric ceramic element including the piezoelectric ceramic of the present invention and electrodes provided therewith.
In the piezoelectric ceramic obtained by such a production method, the content of unreacted Pb is low, and when the piezoelectric ceramic is used for a piezoelectric element, the piezoelectric characteristics can be greatly improved. In the piezoelectric ceramic element using the piezoelectric ceramic, the piezoelectric characteristics according to the piezoelectric ceramic element can be greatly improved.